An ATS provides a substantially continuous source of power for specified loads, such as for example motors and elevators, by automatically transferring the line-to-load connection from a normal power source to a secondary power source when the normal power source falls below pre-established thresholds. The substantially continuous source of power may vary from several seconds of power outage to several seconds of power overlap during transfer, depending on the type of ATS employed. The ability for two power sources to be connected to a load at the same time is generally a function of the controllers and drive systems employed by the ATS, which result in the parallel connection being established for only a fraction of a second, and typically less than 100 milliseconds.
Typical ATS devices comprise a plurality of switching contacts (switches), a drive mechanism, electrical terminal connectors, a controller, a display, and logic devices (such as, for example, voltage, current, and frequency sensors and timers), all typically contained within an enclosure. Control of the switching contacts is provided by a microprocessor, which controls the operation of the ATS's components, including the switches, timers, drive systems, and displays. The display provides a means for displaying the status of the ATS's position, the timers, and the available power sources. Control signals from the controller to the timers and drive system, or software logic within the microprocessor itself, are typically provided in a serial arrangement, and typically result in the actuation of one or more switching contacts, thereby transferring the power feed from one power source, such as a primary source (for example, a utility) to another, such as a backup source (for example, a generator).
An extended ATS control system would typically include at least one automatic transfer switch, as previously described, and a remote computer adapted to be in communication with the ATS's controller, thereby providing remote control and programming of the ATS. Programming of the ATS could be accomplished at either the remote computer or at a touchpad interface on the microcontroller itself, thereby providing for automatic transfer switching when predetermined thresholds are met.
Under primary power conditions, the ATS is typically connected to the primary source. However, when a source parameter, such as voltage, drops below a pre-established threshold, such as 80% of nominal voltage, a transfer sequence is initiated. One example of a transfer sequence includes the starting of a backup engine generator set when the primary source below-threshold signal is received at the controller. When the emergency source reaches 90% of rated voltage and 95% of rated frequency, for example, the drive mechanism is energized, causing the main contacts to disconnect the load from the primary source and connect it to the backup source. After the drive mechanism has completed its stroke and is seated, the drive control de-energizes to disconnect the drive solenoid. The transfer switch is now mechanically locked in the backup position. A reverse action occurs when the primary source voltage is restored to a predetermined level, for example 90% of nominal.